Extension Equine Team

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Toxic Weed: Milkweed

Posted: August 22, 2014

The primary toxin, galitoxin, is found in all vegetative parts of the plant. In addition, a group of toxicants known as cardenolides may be responsible for digitalis-like signs that cause or contribute to death.

Milkweed is a native perennial herb with milky sap and
leaves opposite or whorled, simple and entire;the flowers are in umbels, purple
to greenish white; the fruit is a follicle, with numerous seeds, each with a
tuft of silky hairs.

Description. Milkweeds exude a white, milky juice from
broken or cut surfaces. Both narrow-leafed (whorled) and broad-leafed species
exist; the narrow-leafed variety is most toxic. The fruit is a follicle (i.e.,
a capsule filled with numerous seeds); a silky tuft aids spread of seeds by the
wind. The flower is very distinctive: each flower has five sepals and petals
which are strongly deflexed. Extending upward from the base of each petal is a
club-shaped or hooded lobe.

Toxic principle. The primary toxic principle, galitoxin, is
of the resinoid class. Galitoxin is found in all vegetative parts of the
plant. In addition, a group of toxicants known as cardenolides may be
responsible for digitalis-like signs that cause or contribute to death. In
general, it appears that the broad-leaved species produce cardiotoxic and GI
effects while the narrow-leaved species are more commonly neurotoxic.Dosages of whorled milkweed as low as 0.1 % - 0.5% of the animal's body
weight may cause toxicosis and, possibly, death. Cattle, sheep and horses are
most susceptible. Toxicity is not lost when the plant is dried. Therefore,
contaminated hay is potentially toxic.

Clinical signs include profuse salivation, incoordination, violent seizures,
bloating in ruminants and colic in horses. Early signs are followed by
bradycardia or tachycardia, arrhythmias, hypotension and hypothermia. Death may
occur from 1-3 days after ingestion of the milkweed.